This invention relates to planet gear speed-reducing starter, and more particularly to an improvement of the mechanism in a planet gear speed-reducing starter which is adapted to receive the impact stress which occurs with the power transmission mechanism thereof.
A conventional planet gear speed-reducing starter of this type is as shown in FIG. 6 (cf. Japanese Utility Model (OPI) Application No. 74050/1988 (the term "OPI" as used herein means an "unexamined published application"). In FIG. 6, reference numeral 1 designates the armature of a DC motor; 2, the rotary shaft of the armature 1; 3, a commutator mounted on the rear end portion of the rotary shaft 2; 4, brushes and their holders which are secured to the rear bracket 6 of the motor with bolts 5; 7, a bearing inserted in a recessed formed in the rear bracket 6, thus supporting the rear end of the armature rotary shaft 2; 8, the yoke of the DC motor; and 8a, permanent magnets fixedly mounted on the inner cylindrical surface of the yoke 8, the permanent magnets 8a forming magnetic fields. One end of the cylindrical yoke 8 is connected to the rear bracket 6, and the other end is fixedly secured to the front bracket 10 in which an internal gear 9 forming a planet gear speed-reducing unit is fitted. The yoke 8 and the front bracket 10 are separated with a center plate 11 from each other. A spur gear 12 is formed on the front end portion of the armature rotary shaft 2, and is engaged with planetary gears 13, which are supported through bearings 14 by support pins 15. The support pins 15 are fixedly embedded in a flange 16 which forms the arm of the planet gear speed-reducing unit, and which is secured to an output rotary shaft 17. The output rotary shaft 17 is supported by a sleeve bearing 18 fitted in the inner cylindrical surface of a protrusion extended from the internal gear 9. A sleeve bearing 19 is fitted in a recess formed in the rear end of the output rotary shaft 17, thus supporting the front end portion of the armature rotary shaft 2. A steel ball 20 is provided between the front end face of the armature rotary shaft 2 and the output rotary shaft 17 so as to transmit the thrust load. A helical spline gear 21 is formed on the output rotary shaft 17, and an overrunning clutch 22 is engaged with the helical spline gear 21 in such a manner that it is slidable back and forth. A pinion 24 is coupled to the overrunning clutch 22. The amount of axial movement of the pinion 22 is regulated by a stopper 23. The output rotary shaft 17 is further supported through a sleeve bearing 25 by the front end portion of the front bracket 10. More specifically, the sleeve bearing 25 is fitted in the cylindrical wall of a hole formed in the front end portion of the front bracket 10, and is mounted on the front end portion of the output rotary shaft so as to support the latter. A lever 26 of resin is swingably mounted on the front bracket with its one end coupled to the plunger 28 of an electromagnetic switch 27 and with the other end portion coupled to the rear end portion of the overrunning clutch 22. The electromagnetic switch 27 is connected to the brush 4 through a lead wire 29 one terminal of which is connected to the cap 31 with a nut 30. The center plate 11 has through-holes 32 to lead the heat generated inside the armature toward the front bracket, thereby to prevent the rise of temperature inside the yoke 8.
When the key switch (not shown) is turned on, the electromagnetic switch 27 is energized, so that voltage is applied to the brush 4 through the lead wire 29, and therefore the armature 1 produces torque. The rotation of the armature is transmitted through the planet gear speed-reducing unit to the overrunning clutch 22. On the other hand, upon energization of the electromagnetic switch 27, the plunger 28 is moved to the left in FIG. 6. The movement of the plunger 28 is transmitted through the lever 26 to the overrunning clutch 22. As a result, the overrunning clutch 22 is moved forwardly (to the right in FIG. 6), and the pinion 24 is engaged with the ring gear (not shown) of the engine (not shown).
On the other hand, FIG. 7 shows one example of a conventional planet gear speed-reducing starter having a mechanism adapted to lessen the impact load which may occur with its speed reducer is as shown in FIG. 7 (cf. Japanese Utility Patent (OPI) Application No. 45081/1987). As shown in FIG. 7, a ring gear 29 has a helical spline gear 30 on its outer periphery, and an internal gear 31 on its inner periphery which is engaged with planetary gears 13. A front bracket 10 has a helical spring gear 33 which is formed in the inner wall of its stepped portion 10a so as to engage with the above-described helical spline gear 30 of the ring gear 29. An auxiliary plate 34 is fitted in the stepped portion 10a . The output rotary shaft 17 is supported through a sleeve bearing 18 fitted in the cylindrical hole of the auxiliary plate 34. An elastic ring 35 of rubber is interposed between the ring gear 29 and the auxiliary plate 34. The elastic ring 35 is adapted to elastically receive the force of the ring gear 29 which is threadably moved on the helical spring gear 33 axially forwardly (to the right in FIG. 7) by the reaction of pressure attributing to the inscribed engaging motion of the planetary gears 13. The other arrangements are the same as those in the conventional starter described with reference to FIG. 6.
In the starter shown in FIG. 7, the reaction of pressure attributing to the rotation of the planetary gears 13 is elastically received by the elastic ring 35 as the ring gear 29 is threadably moved, so that the force of impact occurring with the power system of the starter is smoothly received.
In the conventional starter shown in FIG. 6 (Japanese Utility Patent (OPI) No. 74050/1988) the load torque is absorbed only by the rigidity in deflection of the shafts (or the rotary shaft 2, and the output rotary shaft 17) and the brackets (such as the front bracket 10) and by the twist of the overrunning clutch 22. If torque occurs which is larger in energy than the absorption, then the torque cannot be absorbed, thus damaging the engine ring gear and some components of the starter. Such large torque occurs when the starter is operated again as in the case of an engine stall; that is, it occurs during the inertial rotation of the engine or starter, so that the two items large in the energy of rotation collide with each other, thus causing a great impact.
On the other hand, in the conventional starter shown in FIG. 7 (Japanese Utility Patent (OPI) No. 45081/1987, the elastic ring 35 is provided to elastically receive the force of the ring gear 29 which is threadably moved, thereby to absorb the impact. However, the starter suffers from the difficulties that it is unavoidably bulky and intricate in construction because of the possible capacity and fluctuation in characteristic of the elastic member 35, and it is rather unstable in impact absorbing operation.